To obtain a seismic isolator which can be installed in a building having a small load-carrying capacity of a floor, and which can improve a seismic isolation effect even further at the time of occurrence of an earthquake.

The seismic isolator includes a movable-load supporting part which rolls on a rail 40 and which can move a frame 12 in one direction. The movable-load supporting part is arranged at regular intervals in a plurality of movement-direction positions of a frame in the direction orthogonal to the movement direction of the frame. Then an auxiliary load supporting part 8 is arranged in the midsection of the arrangement interval of the movable-load supporting part in two movement-direction positions of the frame in the direction orthogonal to the movement direction of the frame. The auxiliary load supporting part 8 includes a roller 81 which rolls on a base material 85. An auxiliary wheel 24 is arranged in a plurality of position of the frame in the movement direction, and the auxiliary wheel 24 supports the frame by being brought into contact with the floor surface when a wheel 14 on one side in the movement direction turns dislodged from the rail 40. When the frame moves because of an earthquake to cause the wheel of the movable-load supporting part placed in the front in the movement direction and the roller of the auxiliary load supporting part to turn dislodged from the rail and the base material, the auxiliary wheel placed in the front in the movement direction is brought into contact with the floor surface and supports the load of the frame.